Question

I have the following code for Shuffle so far,

    public static IList<T> Shuffle<T>(this IList<T> list)
    {
        var rnd = new Random();
        return list.OrderBy(element => rnd.Next());
    }

and i use it like,

    list = list.Shuffle();

i want to be able to use it like

    list.Shuffle(); // like list.Reverse();

so basically i want to shuffle by reference

I tried the following code,

    public static void Shuffle<T>(this ref IList<T> list)
    {
        var rnd = new Random();
        list.OrderBy(element => rnd.Next());
    }

but it doesn’t work.

Any help is greatly appreciated!

Answer 1

I wrote this quite some time ago. Probably worth checking that it is true to the linked algorithm. These sorts of thing are notoriously easy to get subtly wrong.

    //Fisher-Yates_shuffle http://en.wikipedia.org/wiki/Fisher-Yates_shuffle
    private static readonly ThreadLocal<Random> RandomThreadLocal =
        new ThreadLocal<Random>(() => new Random());
    public static void Shuffle<T>(this IList<T> list, int seed = -1)
    {
        var r = seed >= 0 ? new Random(seed) : RandomThreadLocal.Value;
        var len = list.Count;
        for (var i = len - 1; i >= 1; --i)
        {
            var j = r.Next(i);
            var tmp = list[i];
            list[i] = list[j];
            list[j] = tmp;
        }
    }

As per comments below, could be overloaded for more flexibility:

    private static readonly ThreadLocal<Random> RandomThreadLocal =
        new ThreadLocal<Random>(() => new Random());
    public static void Shuffle<T>(this IList<T> list, int seed)
    {
        list.Shuffle(new Random(seed));
    }

    public static void Shuffle<T>(this IList<T> list)
    {
        list.Shuffle(null);
    }

    public static void Shuffle<T>(this IList<T> list, Random rand)
    {
        var r = rand ?? RandomThreadLocal.Value;

        var len = list.Count;
        for (var i = len - 1; i >= 1; --i)
        {
            var j = r.Next(i);
            var tmp = list[i];
            list[i] = list[j];
            list[j] = tmp;
        }
    }